Anyone hear of a German cartridge --- 8,6?

Not 8.6, but I’ve heard of 8.5 Rabb, a 338-280AI

There's a double on Gunbroker. It's an over and under 16 gauge over a barrel marked 8,6. I never heard of a 8,6

One of the numerous +- 9mm rimmed cartridge back when. Most can be made from something and shot w/cast bullets.
Krauts used bore diameter (sometimes)(mostly)(depending)
Most 8mms are marked 7.6 or 7.